Backflow preventer apparatus are well known in the art and are used in water distribution systems to prevent contamination of the potable water portion by preventing backflow of liquid from the non-potable portion beyond a certain point in the system. Two conditions tend to produce backflow in a water distribution system. The first condition which could cause backflow is known as "back siphonage" and this occurs in the potable supply pipe or main when its pressure drops to cause a vacuum or partial vacuum in that portion of the system. This will cause a backflow of liquid from the service pipe and if the liquid in the service pipe is contaminated, it will also contaminate the potable water supply. The second condition to cause backflow is a condition occurring in the service pipe or nonpotable portion of the system. This condition results in a back pressure condition when the pressure in the nonpotable service pipe exceeds that in the potable supply pipe.
The backflow preventer apparatus is usually installed with a gate valve on either side of the same inbetween a main or supply pipe for potable water and a service line which may be subjected to contaminants and thus be nonpotable water. Installations can be made at inlets to factories such as chemical plants and the like, or inlets to institutions, hotels or any large building complex.
Prior relief valve assemblies used with backflow preventer systems as described above have usually been attached to the side of the backflow preventer body member and normally operated on a vertical axis. Additionally, the prior relief valve assemblies, while being operable in response to a change in the differential pressure across one of the check valves, have had a complicated piping system which required a separate connection to the zone or downstream service pipe for applying this pressure to one side of the diaphragm means therein. More importantly, the passage for dumping the liquid to atmosphere from the zone or the downstream service pipe in prior relief valve assemblies was so arranged that air could not flow in a reverse flow through the passage into the zone to as assist in quickly evacuating the zone.
In situations where malfunctions occurred in the prior relief valve assemblies, it was difficult to repair the relief valve assembly in the field as normally the entire relief valve assembly had to be removed from the backflow preventer apparatus. There was no easy means for servicing the valve element or valve seat in the relief valve assembly.